1. Field of the Invention
The present invention relates to a circuit for detecting variations in the load at the antenna terminal of the transmitter front end of a portable terminal, etc., and more particularly to a detector circuit for detecting the reflection amplitude and phase at the antenna end. The present invention also relates to a semiconductor device using such a circuit.
2. Background Art
The transmitter front end of a wireless terminal generally includes a directional coupler for monitoring the transmission power level, etc. This directional coupler is inserted, e.g., between the transmission power amplifier and the antenna. In such configurations, the directional coupler is used to monitor the output level (or output power) of the amplifier.
A well known transmitter front end will be described with reference to FIG. 21. A semiconductor device 500 serving as a transmitter front end includes a GaAs-based power amplifier (PA) 508, a directional coupler, an RF-IC 510 (an IC for generating a modulated transmission signal), and an antenna 506. The directional coupler includes a main line 502 and a coupled line 504. The main line 502 has an input terminal #IN and an output terminal #OUT. The coupled line 504 has a coupled terminal #C1 for monitoring the output voltage and an isolated terminal #C2.
FIG. 22 shows variations in the load impedance at the antenna end as indicated by four points A to D. FIG. 23 shows an example of how the RF characteristics of the power amplifier vary with variation in the load at the antenna end. Specifically, FIG. 23 shows the RF characteristics of the power amplifier 508 measured when the phase of the load (a tuner) was changed under the condition that VSWR=6:1. It should be noted that the tuner was used as the load in order to simulate variations in the actual load at the antenna end. This diagram plots the power gain of the power amplifier 508, the operating current Ic2 of the final stage Tr (transistor), and the ACLR (Adjacent Channel Power Leakage), which indicates the distortion characteristics of the amplifier, when the output power of the amplifier is maintained constant (e.g., 27 dBm≈0.5 W).
As shown in FIG. 23, the ACLR is highest (i.e., worst) at the phase at point A. The ACLR at the phase at point B, which is 180° from the phase at point A, is also high and close to the ACLR at point A. The operating current Ic2 of the final stage Tr is minimized at point A and maximized at point B. As for the gain, near-optimum gain matching (i.e., a condition where the gain is high and the operating current is low) is obtained at point A, and near-optimum output power matching (i.e., a condition where the maximum output power is high, but the gain is low and the operating current is high) is obtained at point B.
As in this example, the distortion characteristics of the power amplifier may be degraded due to variation in the load at the antenna end. Particularly it is important and necessary to prevent degradation of the distortion characteristics of CDMA power amplifiers. Conventional methods for preventing degradation of the distortion characteristics due to variation in the load include the use of an isolator (a device which allows power to pass from the input IN to the output OUT but prevents power from passing from the output OUT to the input IN) instead of a directional coupler, and the use of a power amplifier employing a balanced configuration. These methods, however, increases the manufacturing cost of the front end.